The present invention relates generally to a no-draw press section for a paper machine and a method of pressing a web in such a press section.
More particularly, the present invention relates to a no-draw press section in which a paper web leaving the wire section of the paper machine is supported by a first felt fabric and is conducted between two felt fabrics through a first double felted press nip which is defined by two press rolls having recessed surfaces and in which dewatering from the paper web takes place through both web surfaces. The press section includes a plain surface roll against which at least two single felted press nips are defined, the first of which is located at a given distance from the first double felted press nip. The first felt fabric also serves as a pressing fabric in the first single felted press nip. The paper web travels between the first double felted press nip and the first single felted press nip carried by the first felt fabric and separates therefrom after passing through the first single felted press nip whereupon the web adheres to the surface of the plain surface roll and is transferred on that surface to the second single felted press nip, the latter being provided with its own felt fabric.
The starting point in the development of the press section of the present invention comprises the "Sym-Press" press section manufactured by Valmet Oy of Finland in view of the several years of experience obtained in the operation thereof.
The construction of a "Sym-Press" press section is disclosed in Finnish publication print No. 50651 and U.S. Pat. No. 4,209,361. Generally, the "Sym-Press" press section constitutes a compact, so-called fully closed press section in which a paper web coming from the forming wire of the paper machine is conducted between two felts through a first press nip which is defined between two rolls including recessed surface rolls and/or suction rolls, so that dewatering of the paper web takes place through both of its surfaces. The press section includes a plain surface roll which is provided with at least one doctor device. A second press nip is defined by the plain surface roll and the second of the two rolls defining the first press nip whereby dewatering takes place in the second press nip through the surface of the paper web which faces the second roll of the first press nip. At least one additional press nip is defined between the plain surface central roll which has a larger diameter than that of the other press rolls of the press section and a recessed surface roll, a felt being passed through such additional press nip, the latter being located on a side of the central plain surface roll which is substantially opposite to the location of the second press nip.
Reference is made to U.S. Pat. No. 4,257,844 as well as to articles published in the following magazines "Das Papier" Heft 1 pages 33-44, 1981 and "Norsk Skogindustri" No. 3, page 80, 1974, with respect to the state of the art relating to the present invention.
A modification of the "Sym-Press" press section described above is described in the last mentioned publications. In such modification the suction roll of the "Sym-Press" does not define a press nip with the plain surface central roll, a first double-felted press nip of the press section being arranged in connection with this suction roll or preceding it and in which dewatering the web takes place in two directions. A recessed surface press roll is substituted for a suction roll of the "Sym-Press" and defines a second press nip of the press section in conjunction with the plain surface central roll. The third press nip is formed against the plain surface central roll on a side thereof which is substantially opposite to the second press nip.
It has been necessary to use a suction roll either as a press roll or as a roll on which the web is carried by a pick-up felt to change the direction of the felt run upwardly towards the second nip. The use of a suction roll or other equivalent suction device has several considerable disadvantages discussed in detail below.
More particularly, the perforations of a suction roll may leave a marking on the paper web which detracts from the appearance of the paper and may affect the surface characteristics of the paper as well. Suction rolls are expensive, each requiring an individual drive motor and associated control system. It is well known that the operation of suction rolls generate significant noise levels and, furthermore, large quantities of air are consumed due to the fact that not only does the air which passes through the web and felt enter the suction system but, additionally, the air which arrives in the suction zone of the suction roll in the shell perforations in each revolution must also enter into the suction system. Additionally, the sealing of the suction box of the suction roll causes many difficulties in practice.
As is well known, a suction roll comprises a rotating perforated cylindrical shell and a stationary suction box situated within the shell which faces and sealingly engages by means of sealing elements the inner side of the cylindrical shell. The suction box generally extends axially from one end of the shell to the other end and has a suction width of about 100-500 mm. The suction box is connected to a suction system so that a flow of air is obtained through the shell perforations on that area thereof which is in communication with the suction box while the roll is rotating. As noted above, suction rolls are expensive components of a paper machine resulting from the fact that the drilling of the shell is a difficult task, among other reasons. The perforations reduce the strength of the shell and, therefore, special metal alloys must be used in the construction of the roll shell and the latter must have a relatively large thickness, all contributing to high material costs.
The amount of air carried in the shell perforations into the suction zone and which therefore enter the suction system has been found to be unexpectedly great in modern high speed paper machines. It follows that the higher the speed of the paper machine, the greater will be the proportion of "hole air" which enters the suction system together with the drying air. This proportion is even further increased by the fact that with increasing machine speeds, the roll shells must be of even greater thickness to provide increased strength, it being understood that the amount of "hole air" is proportional to the thickness of the roll shell.
As also pointed out above, another drawback encountered in the operation of suction rolls is the generation of high noise levels which can cause severe health risks for the operators if certain measures are not taken to avoid such noise. The generation of such high noise levels results from the fact that the perforations formed in the suction roll shell act as whistles. In other words, as the perforations which are subjected to the vacuum in the suction zone travel beyond the suction zone, the same are abruptly filled with air thereby causing a loud whistling noise having a basic frequency equivalent to the acoustical resonating frequency of the hole. The whistle system constituted by the multitude of roll perforations often creates a noise whose level exceeds the pain limit of the human ear. Although attempts have been made to attenuate this noise level by various arrangements, such as by employing a suitable drilling pattern for the perforations, a satisfactory attenuation of this noise has not been achieved in practice.
Another disadvantage in the use of suction rolls is that it is often desirable to provide deflection compensation, especially when such suction rolls are utilized as press rolls. However, the provision of such deflection compensation is not possible as a rule since the suction roll shell is perforated and/or due to the fact that the interior of the roll is occupied by the suction box to such an extent that it is not possible to accommodate conventional deflection compensation apparatus in the roll interior.
Further pertaining to the state of the art relating to the present invention, reference is made to U.S. Pat. No. 4,192,711 in which a method is disclosed for detaching a paper web from a forming wire and conducting it in a so-called closed, no-draw conduction to the press section and for accomplishing a dewatering pressing process. The method disclosed in this patent basically comprises the following steps in sequence:
(a) a felt is conducted onto the web lying on the forming wire, which felt is conducted over the suction slot or slots of a transfer suction box, the web being subjected to a suction effect and the direction of the run of the felt and of the web lying thereon deviated with respect to the run of the forming wire;
(b) the web carried by the felt is conducted around a grooved and/or perforated roll located within the loop of that felt over a substantially large sector on which the web is subjected to an external steam treatment by which the web (and possibly the felt underneath the web) on the roll is heated, the web being supported externally during its change of direction on the roll;
(c) the heated web is then conducted on the felt into the first press nip in which the web is pressed between a recessed surface roll and a plain surface roll for the purpose of dewatering; and
(d) after detachment from the felt, the web is conducted onto the surface of the plain surface roll.